1. Field of the Invention
The present invention relates to a secondary battery charging circuit and, more particularly, to a charging circuit which employs a system for controlling a charging operation by detecting the temperature of a secondary battery upon a charging operation.
2. Description of the Related Art
A system for charging a secondary battery within a short period of time is generally as follows. When charging is controlled at a voltage lowering to .DELTA.V from a peak voltage charged by fast charging, a secondary battery is charged up to 110% to 120%. On the other hand, when charging is controlled by detecting a battery temperature, a secondary battery is charged up to almost 90% of an electrical capacity of the secondary battery by fast charging, and thereafter, the remaining capacity is charged by a quick or trickle charging operation. The most serious problem in this system is determination of a stop timing of fast charging. When the fast charging operation is unnecessarily continued, the battery temperature is extremely increased. As a result, the battery is considerably loaded, thus shortening the service life of the battery.
Thus, various charging circuits for detecting a point where the temperature of a secondary battery is abruptly increased, and stopping a fast charging operation have been proposed.
As one charging circuit utilizing the above-mentioned system, a system, which utilizes characteristics in that the temperature of a secondary battery is increased in the final period of charging, for detecting the temperature of a secondary battery by a temperature sensor, and controlling a charging operation on the basis of the detection result is known. Examples of this system are as follows.
1) A charging circuit for performing fast charging when the temperature of a secondary battery is within a predetermined range, and a time differential value of temperature is equal to or lower than a predetermined value. In this charging circuit, when the temperature of the secondary battery falls within a predetermined range, and the rate of change in temperature is equal to or smaller than a predetermined value, a fast charging current is supplied.
2) A charging circuit for ending charging when the temperature gradient changes from a negative value or 0 to a positive value. This charging circuit has a temperature monitor circuit for monitoring a temperature rise in accordance with a signal from a temperature sensor incorporated in a battery, and controlling the charging circuit. When the temperature of the battery is abnormally increased, the temperature monitor circuit detects the abnormality of the battery temperature, and controls the charging circuit, thereby preventing overcharging.
3) A charging circuit for detecting the temperature of a secondary battery so as to detect a point where the temperature of the secondary battery is abruptly increased, for comparing the present detection temperature and a temperature detected a predetermined period of time before, for, when the difference (temperature differential value) reaches a given positive value, determining that fast charging is completed. More specifically, an output signal from a temperature sensor is converted into a digital value by an A/D converter to obtain temperature data, and digital calculations are performed to make the above-mentioned decision.
However, when a temperature sensor causes an error, or when a connection error between the temperature sensor and an electronic circuit occurs, the temperature sensor cannot supply a normal temperature detection result to the electronic circuit, and charging end control of the secondary battery can no longer be performed. As a result, the secondary battery is overcharged, and its temperature is abnormally increased. Thus, the battery itself or equipment which uses this battery may be damaged.
In the second example, when a battery such as a nickel-hydrogen battery which generates heat from the beginning of charging, and whose temperature is slightly increased along with charging is to be charged, the temperature gradient turns to a positive value after the battery is charged only slightly, and the charging operation is ended, resulting in a considerably short charging state. When the fast charging operation of the above-mentioned secondary battery is performed when ambient temperature is high, the battery temperature exceeds a predetermined value before the end of charging, and the charging operation is undesirably ended. As a result, the same state as described above may occur.
Furthermore, even when the fast charging operation is started by, e.g., a switch after the power switch of the charging circuit is turned on, noise components generated by an abrupt change in charging current are mixed in, e.g., a differential circuit, and the same erroneous operation as described above may occur.
Since the conventional charging circuit has only one temperature sensor, when a plurality of batteries connected in series with each other are to be charged, a change in temperature of one of the batteries is detected by the temperature sensor to control the charging operation. Therefore, if the batteries include a secondary battery (to be referred to as a battery B hereinafter) having a smaller electrical capacity than that of a secondary battery (to be referred to as a battery A hereinafter) to which the temperature sensor is attached, charge control is not started even when the temperature of the battery B is increased. Only when the temperature of the battery A is increased, and the increase in temperature is detected by the temperature monitor circuit, charge control is started. Therefore, the battery B is overcharged. In particular, when the difference between the electrical capacities of the batteries A and B is large, and the batteries A and B are thermally separated from each other, the overcharging amount of the battery is increased. As a result, the battery B causes an abnormal temperature rise, and may be damaged, or equipment which uses the battery B may be damaged.
As described above, when the charging circuit has only one temperature sensor for detecting the temperature of the secondary battery, and when batteries connected in series with each other are to be charged, if the batteries include a secondary battery having a smaller electrical capacity than that of a secondary battery to which the temperature sensor is attached, the battery having the smaller electrical capacity is undesirably overcharged.
In the third method, if, for example, an A/D converter included in a measurement system has a low resolution, a temperature differential value cannot be precisely obtained. More specifically, if the resolution is low, since a small change in temperature of the secondary battery cannot be detected, a point where the temperature is abruptly increased in the final period of charging cannot often be detected. As a result, the stop timing of the fast charging operation is delayed, and the battery is undesirably overloaded. It is preferable to use a 16-bit A/D converter. However, a high-resolution A/D converter is expensive, and it is difficult to use such an A/D converter in a charging circuit included in electronic equipment such as a personal computer, a personal wordprocessor, or the like in view of cost.
As described above, in the conventional charging circuit which measures, as a temperature differential value, a difference between the present detection temperature of the secondary battery and a temperature detected a predetermined period of time before, and for, when the temperature differential value reaches a predetermined value, stopping the fast charging operation, if the A/D converter or the like in the measurement system has a low resolution, an abrupt increase in temperature of the secondary battery in the final period of charging cannot be detected, resulting in overcharging.
References which describe the above-mentioned related arts are as follows:
1) U.S. Pat. No. 3,852,652 (Published Unexamined Japanese Patent Application No. 50-44432) PA0 2) U.S. Pat. No. 4,006,397 (Examined Japanese Patent Publication No. 56-16631) PA0 3) U.S. Pat. No. 4,045,720 PA0 4) U.S. Pat. No. 4,065,712 PA0 5) U.S. Pat. No. 4,052,656 PA0 6) U.S. Pat. No. 4,670,703 PA0 7) U.S. Pat. No. 4,888,544 PA0 8) Published Unexamined Japanese Patent Application No. 52-112741 PA0 9) Published Unexamined Japanese Patent Application No. 61-161926 PA0 10) Published Unexamined Japanese Patent Application No. 61-221538 PA0 11) Published Unexamined Japanese Patent Application No. 62-193518 PA0 12) Published Unexamined Japanese Patent Application No. 63-76275 PA0 13) Published Unexamined Japanese Patent Application No. 1-138931 PA0 14) Published Unexamined Japanese Patent Application No. 1-185135 PA0 15) Published Unexamined Japanese Patent Application No. 1-186128 PA0 16) Published Unexamined Japanese Utility Model Application No. 3-34638